The present invention relates to the phenomenon known as a fiber fuse and in particular method of designing components to prevent the initiation of a fiber fuse, particularly at fiber terminations.
Optical power levels in optical transmission systems are generally increasing. This is due to a number of factors.
For instance, optical transmission systems, including optical fibers and other optical devices such as polarisation mode dispersion compensation mechanisms and photonic switches, all have attenuation characteristics. Increasing the power of an optical signal provides a better signal to background noise ratio, and allows the signal to be transmitted longer distances over the optical transmission system before optical amplification is required. Advances in laser technology have ensured that higher powered lasers are now more readily and cheaply available, thus allowing a cost effective implementation of high optical power signal generation.
Typical optical transmission systems simultaneously transmit data using a multitude of different wavelengths, each transmission channel having a separate wavelength of light for transmission of the respective optical signal. Increasingly, channels are becoming more closely packed together with regard to wavelength e.g. DWDM (Dense Wavelength Division Multiplexed) systems. Increasing the number of simultaneous optical transmissions at different wavelengths will increase the average optical power being carried by the transmission system.
Many optical systems utilise optical amplifiers comprising optical fiber. An example of this is a Raman amplifier i.e. an amplifier that utilises the Raman effect. Optical amplifiers of this type normally use relatively high power pump lasers for providing the optical power that is utilised to amplify the optical signal power. Current trends indicate it is increasingly likely that Raman amplifiers will be utilised in future telecommunications systems.
Experiments have indicated that high optical powers propagating through fibers can induce an effect referred to as a xe2x80x9cfiber fusexe2x80x9d. The fiber fuse effect, also termed self-propelled self-focusing (SPSF), is a catastrophic damage mechanism. Electronics letters, Jan. 7, 1988, Vol. 24, No. 1, pages 47-48 by R Kashyap and K J Blow and Electronics letters Jan. 5, 1989, Vol. 25, No. 1, Pages 33-34 by D P Hand and T A Birks describe this phenomena in some detail and describe a fiber fuse damage circuit-breaker, and are incorporated herein by reference.
The fiber fuse effect is believed to be initiated by local heating of the fiber. This can lead to a runway thermal effect which, provided the laser power is sufficient, continues until the fiber core melts. A thermal shock wave is created (visible as a bright spot of side-scattered light) that propagates back along the fiber towards the optical power source. This results in the fiber being permanently damaged and unable to guide light.
Propagation velocity is believed to be of the order of tens of meters per second. A fiber fuse occurring in a telecommunications system could be extremely damaging. The side-scattered light could also be dangerous to any onlookers. Additionally, in systems where optical fiber spans (i.e. typically the length between optical fiber amplifiers) are of the order of 80 kilometres, it will be appreciated that if the fiber fuse is not contained, it has the capacity to damage large lengths of optical fiber. This would require replacement of the damaged fiber. If the fiber fuse is able to propagate into optical processing equipment, such as an amplifier or pump laser, the fiber fuse can result in damage to very expensive network components.
It is therefore desirable to limit the damage caused by fiber fuses or to prevent initiation of a fiber fuse. As mentioned above, it has been proposed that the initiation of a fiber fuse results from local heating of the fiber. How this local heating is initiated has not been fully understood, although it has been recognised that a fiber fuse may be initiated at the site of fiber damage, such as a fiber break.
The fiber fuse effect is also discussed in the assignee""s copending U.S. patent application Ser. No. 09/544,362, filed Apr. 6, 2000 entitled xe2x80x9cFiber Fuse Protectionxe2x80x9d which is incorporated herein by way of reference material.
The invention stems from the recognition that fiber terminations provide one location where the initiation of a fiber fuse is more likely than at other locations. It has also been recognised that a beam expander can be used to halt the propagation of the fiber fuse, as described in the article of Electronics letters Jan. 5, 1989, reference more fully above.
However, until now, there has not been a detailed analysis of the conditions under which a fiber fuse will be initiated, nor the conditions which permit the propagation of the fiber fuse to be arrested. There is a need for an understanding of these conditions to enable optimum components to be designed which can halt the fiber fuse travel or which can prevent the fuse starting.
This invention concerns specifically fiber terminations. As one example, these are used in optical transmission systems as a so-called xe2x80x9cbeam dumpxe2x80x9d for unabsorbed pump light in an amplifier. Thus, pump light injected into an amplifier which is not absorbed by the amplifier fiber core passes beyond the amplifier, and must be xe2x80x9cdumpedxe2x80x9d to prevent interference outside the amplifier. This is achieved by providing wavelength-dependent routing to the xe2x80x9cbeam dumpxe2x80x9d, which may for example comprise a copper absorber. The pump powers used in amplifiers can be significant, particularly when the Raman amplification effect is being used. Therefore, significant heating can occur at the beam dump, which is one possible cause of the initiation of a fiber fuse.
Other components provide termination of a fiber, for example where signals on two fibers are to be combined, in couplers, combiners or tap devices. In each case, the termination can provide an increased risk of a fiber fuse being generated.
The invention is based on the realisation that a fiber fuse can only be initiated within a fiber when the fiber is carrying power greater than a threshold power. This threshold power is a function of the fiber characteristics, and this understanding enables components for preventing fiber fuse initiation to be designed, by ensuring that they have a higher power threshold.
In a first aspect, the present invention provides an optical termination element for terminating an optical fiber carrying a signal of first maximum power level, comprising a termination fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which is greater than the first maximum power level, the values of the core diameter and the higher mode cutoff wavelength of the termination fiber defining the threshold power level.
The invention provides a termination component which has a threshold power level (below which a fiber fuse can not be started) which exceeds the maximum power to be provided to the termination. A fiber fuse can not therefore be started. The invention is based on the recognition that there is such a threshold power and that the value of the threshold power is a function of the core diameter and the higher mode cutoff wavelength of a fiber. For large core diameters, an increase in core diameter increases the threshold power. This is considered to result from reduced xe2x80x9cthermal leasingxe2x80x9d. This is one phenomenon which can be used to explain the propagation of the thermal fuse, and is based on the idea that a local fuse location is imaged to a focal point within the core at an adjacent location, at which a fuse is created. Increasing the core diameter, and therefore the mode field diameter, increases the size of these focal points and thereby reduces the intensity. For small core diameters, it is believed that heat dissipates more readily to the cladding, so that as the core diameter is reduced, the threshold also increases. Thus, the threshold power has a minimum value with respect to core diameter (for a fiber of constant cutoff wavelength).
The optical fiber to be terminated preferably comprises single mode fiber and the termination fiber comprises multimode fiber, for example a silica rod having an outer diameter corresponding to the outer diameter of the cladding of the fiber to be terminated. The multimode fiber causes the light emerging from the single mode core to diffract to larger spot sizes while being fully enclosed in the termination fiber. This removes the location of any potential heat generation from the single mode core.
In accordance with a second aspect of the invention, there is provided an optical element for processing optical signals, comprising an optical fiber to be terminated carrying a signal of first maximum power level and an optical termination element for terminating the optical fiber, the termination element comprising a termination fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which is greater than the first maximum power level, the values of the core diameter and the higher mode cutoff wavelength of the termination fiber defining the threshold power level.
This optical element thus uses a termination element of the invention. The optical element may comprise a coupler, combiner, beam dump or tap device.
The invention is particularly suitable for use in amplifier designs, in which there is unused high power pump light to be xe2x80x9cdumpedxe2x80x9d. Therefore, in accordance with a third aspect of the invention, there is provided an optical amplifier compressing amplification fiber and a pump source, a first coupler being provided for routing signals from the pump source to a first end of the amplification fiber and a second coupler being provided for routing pump source signals from a second end of the amplification fiber to a termination element using a first fiber, wherein the termination element comprises a termination fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which is greater than the maximum pump source power level routed to the termination element, the values of the core diameter and the higher mode cutoff wavelength of the termination fiber defining the threshold power level.
The amplification fiber may comprise rare earth doped fiber or transmission fiber. In the latter case amplification is by means of the Raman effect.
In accordance with a fourth aspect of the invention there is provided a fiber laser comprising amplification fiber and a pump source, a first coupler being provided for routing signals from the pump source to the amplification fiber and reflectors being provided at the ends of the amplification fiber, a termination element being provided for terminating unused pump power which comprises a termination fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which is greater than the maximum power routed to the termination element, the values of the core diameter and the higher mode cutoff wavelength of the termination fiber defining the threshold power level.
The invention also provided a method of designing an optical termination element for terminating an optical fiber carrying a signal of first maximum power level, comprising:
selecting a threshold power level which is greater than the first maximum power level; and
selecting a termination fiber which is unable to propagate a fiber fuse when the signal power is below the threshold power level, by selecting values of the core diameter and the higher mode cutoff wavelength.